A paper web formed in a paper machine has varying width and its edges are uneven as regards straightness, thickness and grammage. It is therefore necessary to edge-trim the paper web during some part of the manufacturing process. A number of different techniques have been proposed in order to achieve a paper web with straight edges and having predetermined width.
One known technique for edge-trimming a formed paper web is to trim the edges from the web when the finished, reeled paper web is rewound in a rewinder. The waste of dried paper that is formed during this edge-trimming is extremely voluminous, which complicates its collection and removal to be re-dissolved in water and returned to the paper manufacturing process.
According to another known technology, edge-trimming occurs in the paper machine at a point between the drying section and the reel-up, with the aid of high-pressure water jets, air jets, rotating knives, or rotating saw blades. This technology thus requires installation of extra equipment in the extremely limited space available between the drying section and the reel-up.
According to yet other known technology, the edge trimming occurs in the wet section of the paper machine, before the drying section. For example, it is known from U.S. Pat. Nos. 2,686,463 and 2,709,398 to trim the edges of a paper web on a fourdrinier wire with the aid of water jets so that it is divided into an edge-trimmed web, and couch trimmings, after which the edge-trimmed web is removed from the wire for transfer to the drying section, while the couch trimmings are prevented from reaching the drying section. However, it has been found that this technology cannot be used for paper grades having low grammages, such as soft paper, since the couch trimmings adhere to the pickup felt used for removing the edge-trimmed web and therefore tend to accompany it to the drying section.
In a twin wire former it is in practice known to perform edge trimming of the paper web while it is supported by one of the forming fabrics at a point immediately before the pickup means. Water jets are generally used for edge trimming, these jets encountering the paper web with relatively high pressure to produce an edge trimming and an edge-trimmed web. One drawback is that, after having divided the paper web, the water jets spread, splashing water around the whole area. If the supporting fabric is a felt, as in the case of a crescent former, there is considerable risk of fibers and fiber fragments being pressed into the felt. The pressure of the water jets must be limited in order to reduce the risk of their damaging the felt. In spite of these measures, however, the water jets gradually cause wear on the felt and this wear may affect the paper web, causing edge rupture and consequent risk of web rupture. The felt also becomes wetter along the dividing lines formed by the water jets than over the rest of the felt, which may cause problems since the drying cylinder becomes wetter opposite these dividing lines than over other parts of the outer surface of the drying cylinder, thereby incurring problems with corrosion and deposits. The water jets from the jet tubes may also easily cause the paper edge to thicken around the water jets and this thickening may result in deterioration of the adhesion to the drying cylinder at the edge portions of the paper web. The outer surface also becomes hotter opposite the dividing lines, which may cause the paper web to become detached from the outer surface. To solve these problems and provide a paper web with straight edges and of a predetermined width in a crescent former, therefore, the outer forming fabric may be provided with impermeable edge portions, as described in U.S. Pat. No. 3,652,390. The inner parallel edges of said edge portions facing each other thus determine the width of the finished paper web, and the width is chosen dependent on the grade of soft paper to be manufactured. When the manufacture of soft paper is to be changed from one grade to another, for example from tissue to toweling paper, the width of the finished paper must be changed in order to avoid undesirable losses during conversion of the paper web to the desired final products. To enable such a change in manufacture from one paper type to another, the outer forming fabric of the crescent former must be dismantled and replaced by another forming fabric with a different width between the impermeable edge portions. This exchange is laborious, time-consuming, and entails undesired loss of production. Increased costs for forming fabrics are also incurred, as well as space for their storage.
To solve the problems inherent in the technologies used in the twin-wire former described, EP-0 654 101 proposes cutting a paper web formed in a twin-wire former with a jet cutter at a point on the forming roll where the recently formed paper web and the forming fabrics together enclose the forming roll in a sandwich structure. This technology entails a limitation of the choice of forming wires since the water jets must have sufficient force to pass through the wire and the paper web. There is then a risk of the surface of the opposite felt being damaged. The felt also becomes wetter at the dividing lines formed by the water jets than at other parts of the felt, which may cause problems since the drying cylinder becomes wetter opposite these dividing lines than other parts of the outer surface of the drying cylinder, thus involving problems of corrosion and deposits. The outer surface also becomes hotter opposite the dividing lines, which may cause the paper web to become detached from the outer surface.
U.S. Pat. Nos. 1,279,756, 2,857,822 and 4,560,438 disclose still other examples of methods and apparatus for edge trimming in a paper machine before the drying section.
Slitter-winders are used for dividing a paper web in the-longitudinal direction, in which slitter-winder the narrower webs obtained through longitudinal slitting are wound to reels. Dividing a paper web through longitudinal slitting can also be performed in the paper machine at a point between the drying section and the reel-up using equipment similar to that used in the edge trimming described above, in the same position.